CD1d-restricted T cells (or "NKT cells") have been reported to regulate an extremely diverse set of immunologic responses and diseases. Dysfunction of these T cells is clearly correlated with the development of autoimmunity, and in particular autoimmune diabetes. Despite the importance of CD1d-restricted T cells in this disease, how these T cells function normally and the exact nature of the disease-associated defects remains unclear. In this regard, potential regulatory functions that would be predicted to have significant impact on type 1 diabetes include recently described critical interactions of CD1d-restricted T cells with dendritic cells and the activation-induced secretion of Th1 and Th2 cytokines. Th2 cytokine secretion by CD1d-restricted T cells has been associated with protection from autoimmune diabetes in murine models. Conversely, CD1d-restricted T cells cloned from patients with type 1 diabetes were found, amongst other defects, to have an extreme Th1 cytokine bias. Importantly, recent work has demonstrated that in normal human volunteers, the CD4+ CD1d-restricted subset is responsible for Th2 cytokine production in vivo, whereas the CD4- (or "DN") subset were strongly biased towards the secretion of Th1 cytokines and expressed greater levels of proteins with cytotoxic function. Thus, CD4+ CD1d-restricted T cells might serve to prevent progression to diabetes whilst DN CD1d-restricted T cells might promote pro-inflammatory responses. To test the hypothesis of distinct effector function for these two subsets we propose to compare diabetes patients, at risk donors, and control donors in the following specific aims: 1. Phenotypic analyses of PBMC samples, using CD1d tetramers and CD1d-restricted T cell specific mAbs, to identify changes in the CD1d-restricted T cell population and to assess whether these correlate with the acquisition of clinical indicators of progression towards type 1 diabetes. 2. Functional analyses (e.g. cytokine production, cytolytic function) of CD1d-restricted T cells in PBMC samples, and of short term polyclonal CD1d-restricted T cell lines and clones. 3. Analyses of self antigen reactivity of CD1d-restricted T cell clones and lines derived in Aim 2.